Dendritic cells (DC) are professional antigen-presenting cells of the immune system. They have the ability to take up, process and present antigens (Ag) to resting lymphocytes in draining lymph nodes. The importance of DC in initiation of T cell immunity and tolerance is firmly established (Steinman et al., 2003). Functionally mature DC are highly efficient at priming adaptive immune responses against viruses, pathogens and endogenous tumors (Banchereau and Steinman, 1998); whereas steady state “immature” DC are capable of uptaking and presenting self-Ag to tolerize autoreactive T cells (Wilson et al., 2003; Bonifaz et al., 2002; Hawiger et al., 2001; Scheinecker et al., 2002; Steinman et al., 2000). Phenotypic analysis of surface expression of co-stimulatory molecules has been routinely used in defining mature and “immature” DC. The ability to use DC in the induction of antigen (Ag)-specific tolerance, Ag-specific immunity or specific differentiation of T helper subsets holds great promises in DC-based immunotherapy of various diseases such as cancer, viral infections, allergy, as well as autoimmunity. The increasing number of co-stimulatory molecules identified to date, however, highlights the complex regulation of co-stimulatory signals.
RNA interference (RNAi) is an innate cellular process that involves multiple RNA-protein interactions (Fire, 1999; Hannon, 2002). Its gene silencing activity is activated when a double-stranded RNA (dsRNA) molecule of greater than 19 duplex nucleotides enters the cells, causing degradation of both the dsRNA and single stranded RNA (endogenous mRNA) of identical sequences (Fire, 1999; Hannon, 2002). Transient transfection of short synthetic dsRNA in DC demonstrated the feasibility of applying this technique in studying DC functions (Li et al., 2004; Gu et al., 2006; Hill et al., 2003; Laderach et al., 2003; Liu et al., 2004; Orabona et al., 2005; Li et al., 2007). A single lentiviral vector system has been established to stably express short hairpin RNA (shRNA) to induce RNA interference (RNAi) in cell lines and primary T cells (An et al., 2003; Qin et al., 2003).